1. Field of the Invention
The present invention relates to a position measuring system, having a graduation support with a measuring graduation which can be scanned by light, having a planar waveguide, by which light generated by a light source is conducted to the measuring graduation, wherein the light is reflected at the surfaces of the planar waveguide, and having a scanning unit for scanning the measuring graduation, which has at least one light- sensitive scanning area to which the light modulated by the measuring graduation is conducted.
2. Discussion of Related Art
An angle measuring device is known from EP 0 589 477 A2, which includes a light source, a disk, which is provided with a measuring graduation and can be connected, fixed against relative rotation, to a shaft to be measured, as well as a photodetector. For realizing the angle measurement, the light coupled out by the light source is initially reflected at a first location on the measuring graduation. Subsequently the reflected light is conducted by a waveguide to another location on the measuring graduation, where it is again reflected and from where it reaches the photodetector.
A photoelectric position measuring device for measuring the relative position of two objects is known from EP 0 426 882 B1, wherein a graduation support with a measuring graduation made of transparent strips and light-reflecting strips is scanned by a scanning unit, which has an illumination unit, a scanning plate with at least one scanning graduation, as well as photodetectors. In this case, the bundle of light rays emanating from the illuminating unit is split into a partial bundle of rays passing through the transparent strips for acting on a first photodetector, and into a partial bundle of rays reflected by the light-reflecting strips for acting on a second photodetector, and the position measuring device is designed in such a way that the partial bundle of rays passing through the measuring graduation is modulated by the scanning graduation, and the partial bundle of rays reflected by the measuring graduation is not modulated by the scanning graduation. A plan-parallel waveguide, which includes the graduation support of the measuring graduation, is provided for conducting the bundle of light beams emanating from the illumination unit to the measuring graduation.
An object of the present invention is based on providing a position measuring system of the type mentioned at the outset, which is distinguished by an improved delivery of the light to the measuring graduation.
In accordance with the present invention, this object is attained by providing a position measuring system that includes a graduation support having a measuring graduation, a planar waveguide and a light source that generates light that is conducted to the measuring graduation, wherein the light is reflected at surfaces of the planar waveguide. A scanning unit for scanning the measuring graduation that includes a light-sensitive scanning area to which light modulated by the measuring graduation is conducted. The modulated light conducted to the light-sensitive scanning area has at least two different light portions, which differ in a number of reflections at the surfaces of the planar waveguide, and wherein the reflecting of the at least two different light portions occurs prior to reaching the measuring graduation.
In accordance therewith the light conducted to the scanning area has at least two different light components, wherein the light of the different components emanates from the waveguide at different locations and differs in the number of reflections which the light beams of the respective light component undergo prior to reaching the measuring graduation in the planar waveguide.
The design in accordance with the present invention of the position measuring system is particularly advantageous in connection with measuring graduations which can be scanned by an incident light method, i.e. which reflect the light conducted to them. If in such a measuring graduation the scanning unit is arranged at a short distance above the graduation support, the problem arises that the scanning unit makes the illumination of the measuring graduation more difficult. In this case it is made possible by the use of a planar waveguide to conduct the light specifically to the measuring graduation. A planar waveguide is understood to be a waveguide having two planar surfaces located opposite each other, at which the light is reflected while it is conducted along the waveguide.
The solution in accordance with the present invention has the advantage that, based on the different number of reflections which different light rays undergo prior to impinging on the measuring graduation, and thereafter on the same scanning area of the scanning unit, the surface section of the plane waveguide through which the light emanates from the waveguide (to finally reach the scanning area) can be considerably larger than that surface section, through which the light is coupled into the waveguide. Because of this, an area of a large surface of the measuring graduation, as well as a corresponding scanning area of the scanning unit can be completely illuminated by the light, even if this light is coupled into the planar waveguide through a comparatively smaller surface section.
The area of the measuring graduation, which is scanned, or illuminated, over a large surface with a different number of previous reflections in the waveguide, is here understood to be a uniform area which provides the same position information. Thus, this does not relate to the illumination of completely separate sections of a graduation support, which are arranged next to each other, such as for example an incremental track on the one side and a reference bar track on the other, but to the scanning of a uniform (incremental or absolute) measuring graduation.
Several analogous, sinusoidal scanning signals which are offset with respect to each other are generated in incremental position measuring systems, namely as a rule four scanning signals, which are each phase-shifted by 90xc2x0 with respect to each other. These scanning signals are linked together for evaluation. In this regard it is important that they have equal offset portions. This is only assured if all scanning signals are obtained from a common uniform area of the measuring graduation. To compensate for partial contamination of the measuring graduation it is important here that not only one graduation bar of the measuring graduation is a partial component of the generation of each one of the scanning signals, but a plurality of graduation bars, so that they can be averaged. For this reason it is required to illuminate the largest possible scanning area of the measuring graduation which provides a uniform position information, and to derive all scanning signals from this coherent area (xe2x80x9csingle field scanningxe2x80x9d).
The measuring graduation of absolute position measuring systems includes a single-track or multi-track coding with code elements, from which a multi-digit code word is formed. During scanning, the code elements required for forming the code word should be illuminated by a common light source, if possible. The illumination of the largest possible uniform area of the measuring graduation, or of the coding which provides the code word characteristic for a defined position in the measuring direction, is therefore also important here.
The coupling element for coupling the light into the waveguide, as well as the coupling element for coupling the light out from the waveguide, can be integrated in a simple manner into the surface of the waveguide, for example in that these coupling elements are stamped as a single-piece component of the waveguide into its surface. A Fresnel grating, or a prism, for example, are suitable for coupling in the light into the planar waveguide.
In accordance with a variation of the present invention, the light rays of the bundle of light rays coupled into the planar waveguide extend parallel with each other in the waveguide. It is thus possible, on the one hand, to provide the light to the waveguide already as a parallel bundle of light rays (i.e. the parallelization of the light takes place outside the waveguide), or the light is parallelized when being coupled into the waveguide, for example by a Fresnel lens, in which the grating constant is spatially varied (off-axis Fresnel lens).
With this variation of the present invention, a respective portion of the light conducted through the waveguide is put out at at least two locations, which are spaced apart in the direction of the extension of the waveguide, wherein the light is reflected in the waveguide at least once between these two locations. By this it is achieved that the light is coupled out of the waveguide with a different number of previous reflections, and can be subsequently supplied first to the measurement graduation, and then to a scanning area. In the course of this, light rays moving along the same track undergo different amounts of reflections in the waveguide before they are coupled out from it.
In accordance with a further variation of the present invention, the light rays extend in the form of a divergent bundle of rays in the planar waveguide. This can be achieved, for example, in that light is coupled into the planar waveguide by a prism, wherein light of different frequencies is deflected at different angles. In this case the light rays extend in the waveguide at different angles with respect to the direction of extension of the waveguide, so that a different number of reflections of different light rays is achieved by this.
In a preferred embodiment of the present invention, the waveguide is arranged between the scanning area of the scanning unit and the graduation support and includes a scanning plate, whose scanning grating works together with the measuring graduation for modulating the light. In this case, the scanning grating can be simultaneously used for coupling out the light from the waveguide.
In another embodiment of the present invention the waveguide includes the graduation support.
In one way the coupling element for coupling light into the wave guide can be provided on the same surface of the waveguide as the coupling element for coupling out the light from the waveguide. Alternatively, the coupling elements for coupling in and coupling out light into the waveguide, or from the waveguide, are arranged on surfaces of the waveguide, which are located opposite each other. In both cases, both surfaces of the waveguide are available for the coupling elements for coupling in and/or for coupling out light.
Preferably the waveguide is made of glass or of a plastic material.
Further components, for example electrical components, can be provided on the surfaces of the planar waveguide, besides optical components, for example in the form of coupling elements for coupling in or coupling out light. These can be arranged, for example by so-called xe2x80x9cchip-on-glassxe2x80x9d techniques, on the surface of the waveguide. The electrical components can be strip conductors in particular.
Further characteristics and advantages of the invention will become apparent in the course of the following description of exemplary embodiments by the drawings.